(a) Why does presence of excess of lithium make LiCl crystals pink?
(b) A solid with cubic crystal is made of two elements P and Q. Atoms of Q are at the corners of the cube and P at the body-centre. What is the formula of the compound?

(a) In presence of excess of Li, the $C{l}^{-}$ ions leave their lattice sites creating anion vacancies which are occupied by electrons given out by Lithium. Such sites are called F-centres and are responsible for pink colour of the crystal.

(b) No. of atoms of Q per unit cell $=8\times \frac{1}{8}=1$ (at corners)
No. of atoms of P per unit cell = 1 (at body centre)
$\therefore$ Formula of the compound = PQ.